Image sensors with in-pixel lens arrays

ABSTRACT

An image sensor may include an array of pixels. Pixels in the array may include a photodiode that converts incident light into electrical charge and a charge storage region for storing the electrical charge before it is read out from the pixel. Pixels in the array may include a microlens formed over the photodiode that directs light onto the photodiode. Pixels in the array may include an additional array of microlenses between the microlens and the photodiode. The additional array of microlenses may direct light away from the charge storage region to prevent charge stored at the charge storage region from being affected by light that is not incident upon the photodiode. The image sensor may be a backside illuminated image sensor that operates in a global shutter mode.

BACKGROUND

This relates generally to imaging devices, and more particularly, toimaging devices having pixels with charge storage regions.

Image sensors are commonly used in electronic devices such as cellulartelephones, cameras, and computers to capture images. In a typicalarrangement, an electronic device is provided with an array of imagepixels arranged in pixel rows and pixel columns. The image pixelscontain a photodiode for generating charge in response to light.Circuitry is commonly coupled to each pixel column for reading out imagesignals from the image pixels.

Image sensors of this type may include a charge storage region forstoring charge generated by the photodiode before the charge is readout. Light that is incident upon the charge storage region while thecharge is being held before readout may cause an unwanted change in thecharge storage region, thereby causing the readout from the storageregion to inaccurately represent the charge generated by the photodiode.

It would therefore be desirable to provide image sensors with structuresfor directing incident light away from the charge storage region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having an imagesensor and processing circuitry for capturing images using a pixel arrayin accordance with an embodiment of the present invention.

FIG. 2 is a diagram of an illustrative pixel array and associatedreadout circuitry for reading out image signals from the pixel array inaccordance with an embodiment of the present invention.

FIG. 3 is a schematic diagram of an illustrative image sensor pixel inaccordance with an embodiment of the present invention.

FIG. 4 is a cross-sectional side view of an illustrative image sensorpixel having an in-pixel lens array in accordance with an embodiment ofthe present invention.

FIG. 5 is a top view of an illustrative in-pixel lens array of the typeshown in FIG. 4 in accordance with an embodiment of the presentinvention.

FIG. 6 is a cross-sectional side view of a portion of an illustrativeimage sensor pixel having multiple in-pixel lens arrays in accordancewith an embodiment of the present invention.

FIG. 7 is a block diagram of an illustrative image capture and processorsystem employing the embodiments of FIGS. 1-6 in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

Electronic devices such as digital cameras, computers, cellulartelephones, and other electronic devices may include image sensors thatgather incoming light to capture an image. The image sensors may includearrays of image pixels. The pixels in the image sensors may includephotosensitive elements such as photodiodes that convert the incominglight into image signals. Image sensors may have any number of pixels(e.g., hundreds or thousands or more). A typical image sensor may, forexample, have hundreds of thousands or millions of pixels (e.g.,megapixels). Image sensors may include control circuitry such ascircuitry for operating the image pixels and readout circuitry forreading out image signals corresponding to the electric charge generatedby the photosensitive elements.

FIG. 1 is a diagram of an illustrative imaging system such as anelectronic device that uses an image sensor to capture images.Electronic device 10 of FIG. 1 may be a portable electronic device suchas a camera, a cellular telephone, a tablet computer, a webcam, a videocamera, a video surveillance system, an automotive imaging system, avideo gaming system with imaging capabilities, or any other desiredimaging system or device that captures digital image data. Camera module12 may be used to convert incoming light into digital image data. Cameramodule 12 may include one or more lenses 14 and one or morecorresponding image sensors 16. Lenses 14 may include fixed and/oradjustable lenses and may include microlenses formed on an imagingsurface of image sensor 16. During image capture operations, light froma scene may be focused onto image sensor 16 by lenses 14. Image sensor16 may include circuitry for converting analog pixel data intocorresponding digital image data to be provided to storage andprocessing circuitry 18. If desired, camera module 12 may be providedwith an array of lenses 14 and an array of corresponding image sensors16.

Storage and processing circuitry 18 may include one or more integratedcircuits (e.g., image processing circuits, microprocessors, storagedevices such as random-access memory and non-volatile memory, etc.) andmay be implemented using components that are separate from camera module12 and/or that form part of camera module 12 (e.g., circuits that formpart of an integrated circuit that includes image sensors 16 or anintegrated circuit within module 12 that is associated with imagesensors 16). Image data that has been captured by camera module 12 maybe processed and stored using processing circuitry 18 (e.g., using animage processing engine on processing circuitry 18, using an imagingmode selection engine on processing circuitry 18, etc.). Processed imagedata may, if desired, be provided to external equipment (e.g., acomputer, external display, or other device) using wired and/or wirelesscommunications paths coupled to processing circuitry 18.

As shown in FIG. 2, image sensor 16 may include a pixel array 20containing image sensor pixels 30 arranged in rows and columns(sometimes referred to herein as image pixels or pixels) and control andprocessing circuitry 45 (which may include, for example, image signalprocessing circuitry). Array 20 may contain, for example, hundreds orthousands of rows and columns of image sensor pixels 30. Controlcircuitry 45 may be coupled to row control circuitry 47 and imagereadout circuitry 48 (sometimes referred to as column control circuitry,readout circuitry, processing circuitry, or column decoder circuitry).Row control circuitry 47 may receive row addresses from controlcircuitry 45 and supply corresponding row control signals such as reset,row-select, charge transfer, dual conversion gain, and readout controlsignals to pixels 30 over row control lines 49. One or more conductivelines such as column lines 43 may be coupled to each column of pixels 30in array 20. Column lines 43 may be used for reading out image signalsfrom pixels 30 and for supplying bias signals (e.g., bias currents orbias voltages) to pixels 30. If desired, during pixel readoutoperations, a pixel row in array 20 may be selected using row controlcircuitry 47 and image signals generated by image pixels 30 in thatpixel row can be read out along column lines 43.

Image readout circuitry 48 may receive image signals (e.g., analog pixelvalues generated by pixels 30) over column lines 43. Image readoutcircuitry 48 may include sample-and-hold circuitry for sampling andtemporarily storing image signals read out from array 20, amplifiercircuitry, analog-to-digital conversion (ADC) circuitry, bias circuitry,column memory, latch circuitry for selectively enabling or disabling thecolumn circuitry, or other circuitry that is coupled to one or morecolumns of pixels in array 20 for operating pixels 30 and for readingout image signals from pixels 30. ADC circuitry in readout circuitry 48may convert analog pixel values received from array 20 intocorresponding digital pixel values (sometimes referred to as digitalimage data or digital pixel data). Image readout circuitry 48 may supplydigital pixel data to control and processing circuitry 45 and/orprocessor 18 (FIG. 1) for pixels in one or more pixel columns.

If desired, a color filter array may be formed over photosensitiveregions in array 20 so that a desired color filter element in the colorfilter array is formed over an upper surface of the photosensitiveregion of an associated pixel 30. A microlens may be formed over anupper surface of the color filter array to focus incoming light onto thephotosensitive region associated with that pixel 30. Incoming light maybe focused onto the photosensitive region by the microlens and may passthrough the color filter element so that only light of a correspondingcolor is captured at the photosensitive region. If desired, an optionalmasking layer may be interposed between the color filter element and themicrolens for one or more pixels 30 in array 20. In another suitablearrangement, an optional masking layer may be interposed between thecolor filter element and the photosensitive region for one or morepixels 30 in array 20. The masking layers may include metal maskinglayers or other filtering layers that block a portion of the image lightfrom being received at the photosensitive region. The masking layersmay, for example, be provided to some image pixels 30 to adjust theeffective exposure level of corresponding image pixels 30 (e.g., imagepixels 30 having masking layers may capture less light relative to imagepixels 30 without masking layers). If desired, image pixels 30 may beformed without any masking layers.

If desired, pixels 30 in array 20 of FIG. 2 may be provided with anarray of color filter elements that each pass one or more colors oflight. All or some of pixels 30 may be provided with a color filterelement. Color filter elements for pixels 30 may be red color filterelements (e.g., photoresist material that passes red light whilereflecting and/or absorbing other colors of light), blue color filterelements (e.g., photoresist material that passes blue light whilereflecting and/or absorbing other colors of light), and/or green colorfilter elements (e.g., photoresist material that passes green lightwhile reflecting and/or absorbing other colors of light). Color filterelements may also be configured to filter light that is outside thevisible human spectrum. For example, color filter elements may beconfigured to filter ultraviolet or infrared light (e.g., a color filterelement may only allow infrared light or ultraviolet light to reach thephotodiode). Color filter elements may configure image pixel 30 to onlydetect light of a certain wavelength or range of wavelengths (sometimesreferred to herein as a wavelength band) and may be configured to allowmultiple wavelengths of light to pass while blocking light of certainother wavelengths (for example, light having a wavelength thatcorresponds to a certain visible color and/or an infrared or ultravioletwavelength).

Color filter elements that pass two or more colors of light (e.g., twoor more colors of light selected from the group that includes red light,blue light, and green light) are sometimes referred to herein as“broadband” filter elements. For example, yellow color filter elementsthat are configured to pass red and green light and clear color filterelements that are configured to pass red, green, and blue light may bereferred to herein as broadband filter elements or broadband colorfilter elements. Magenta color filter elements that are configured topass red and blue light may be also be referred to herein as broadbandfilter elements or broadband color filter elements. Similarly, imagepixels that include a broadband color filter element (e.g., a yellow,magenta, or clear color filter element) and that are therefore sensitiveto two or more colors of light (e.g., that capture image signals inresponse to detecting two or more colors of light selected from thegroup that includes red light, blue light, and green light) maysometimes be referred to herein as broadband pixels or broadband imagepixels. Image signals generated by broadband image pixels may sometimesbe referred to herein as broadband image signals. Broadband image pixelsmay have a natural sensitivity defined by the material that forms thebroadband color filter element and/or the material that forms the imagesensor pixel (e.g., silicon). In another suitable arrangement, broadbandimage pixels may be formed without any color filter elements. Thesensitivity of broadband image pixels may, if desired, be adjusted forbetter color reproduction and/or noise characteristics through use oflight absorbers such as pigments. In contrast, “colored” pixel may beused herein to refer to image pixels that are primarily sensitive to onecolor of light (e.g., red light, blue light, green light, or light ofany other suitable color). Colored pixels may sometimes be referred toherein as narrowband image pixels because the colored pixels have anarrower spectral response than the broadband image pixels.

If desired, narrowband pixels and/or broadband pixels that are notconfigured to be sensitive to infrared light may be provided with colorfilters incorporating absorbers of NIR radiation. Color filters thatblock near-infrared light may minimize the impact of infrared light oncolor reproduction in illuminants containing both visible and infraredradiation.

As an example, image sensor pixels such as the image pixels in array 20may be provided with a color filter array which allows a single imagesensor to sample red, green, and blue (RGB) light using correspondingred, green, and blue image sensor pixels arranged in a Bayer mosaicpattern. The Bayer mosaic pattern consists of a repeating unit cell oftwo-by-two image pixels, with two green image pixels diagonally oppositeone another and adjacent to a red image pixel diagonally opposite to ablue image pixel. In another suitable example, the green pixels in aBayer pattern are replaced by broadband image pixels having broadbandcolor filter elements. These examples are merely illustrative and, ingeneral, color filter elements of any desired color and in any desiredpattern may be formed over any desired number of image pixels 30.Circuitry in an illustrative image pixel 30 of image pixel array 20 isshown in

FIG. 3. As shown in FIG. 3, pixel 30 may include a photosensitiveelement such as photodiode 22 (sometimes referred to herein asphotodetector 22). A positive pixel power supply voltage (e.g., voltageVaa_pix) may be supplied at positive power supply terminal 33. A groundpower supply voltage (e.g., Vss) may be supplied at ground terminal 32.Incoming light is gathered by photodiode 22 after passing through acolor filter structure. Photodiode 22 converts the light to electricalcharge.

Before an image is acquired, reset control signal RST may be asserted.This turns on reset transistor 28 and resets floating diffusion region27 (sometimes to herein as a first floating diffusion node) to Vaa_pix.The reset control signal RST may then be deasserted to turn off resettransistor 28.

As shown in FIG. 3, pixel 30 may include a charge storage region 26(sometimes referred to herein as a memory node, global shutter storagediode, or charge storage node). Although charge storage region 26 isshown as a storage gate SG in FIG. 3, this is merely illustrative. Ifdesired, charge storage region 26 may be a charge storage diode, acharge storage capacitor, or an additional floating diffusion region(sometimes referred to herein as a second floating diffusion node). Ingeneral, charge storage node 26 may be implemented using a region ofdoped semiconductor (e.g., a doped silicon region formed in a siliconsubstrate by ion implantation, impurity diffusion, or other dopingtechniques). The doped semiconductor region may exhibit a capacitancethat can be used to store charge that has been generated by photodiode22. First charge transfer transistor 24 (sometimes referred to herein asa first transfer gate) may be asserted to transfer charge generated byphotodiode 22 to charge storage region 26. In arrangements in whichimage sensor 16 is operated in a global shutter mode, the chargegenerated by each photodiode 22 in each pixel 30 in array 20 may besimultaneously transferred to the respective charge storage region 26 ineach pixel 30 at the same time (e.g., transfer gate 24 may be pulsedhigh simultaneously for each pixel 30 in array 20).

Once the charge generated by each photodiode 22 in array 20 has beentransferred to a respective charge storage region 26, the readout ofcharge from the charge storage region 26 may proceed in a sequentialrow-by-row manner for each row of pixels in array 20. Charge may betransferred from charge storage region 26 to floating diffusion 27 byasserting second charge transfer transistor 25 (sometimes referred toherein as a second transfer gate). Floating diffusion 27 may beimplemented using a region of doped semiconductor (e.g., a doped siliconregion formed in a silicon substrate by ion implantation, impuritydiffusion, or other doping techniques). The doped semiconductor regionmay exhibit a capacitance that can be used to store charge that has beengenerated by photodiode 22 and transferred from charge storage region26. The signal associated with the stored charge on floating diffusionnode 27 is buffered by source-follower transistor 34. Row selecttransistor 36 connects the source follower transistor 34 to columnoutput line 41.

When it is desired to read out the value of the stored charge onfloating diffusion 27 (i.e., the value of the stored charge that isrepresented by the signal at the source S of transistor 34), row selectcontrol signal RS can be asserted. When signal RS is asserted,transistor 36 turns on and a corresponding signal Vout that isrepresentative of the magnitude of the charge on charge storage node 26is produced on output path 38. In a typical configuration, there arenumerous rows and columns of pixels such as pixel 30 in the image sensorpixel array of a given image sensor. A conductive path such as path 41can be associated with one column of image pixels 30.

When signal RS is asserted in a given of pixel 30, path 41 can be usedto route signal Vout from the pixel 30 to readout circuitry (e.g., 48 inFIG. 2).

If desired, other types of image pixel circuitry may be used toimplement the image pixels of sensors 16. For example, each image sensorpixel 30 may be a three-transistor pixel, a pinned-photodiode pixel withfour transistors, etc. The circuitry of FIG. 3 is merely illustrative.

As described above, every pixel may simultaneously capture an image inan image sensor operating in a global shutter scheme. In a globalshutter scheme, all of the pixels in an image sensor may be resetsimultaneously. A charge storage region 26 is typically incorporatedinto each pixel. The first transfer operation (asserting transfer gateTX0) is then used to simultaneously (globally) transfer the chargecollected in the photodiode of each image pixel to the associatedstorage region 26 to store the charge until the second transferoperation (asserting transfer gate TX1) is performed on a row-by-rowbasis and the charge is read out. With such an arrangement, however,light that is incident upon the charge storage region 26 instead of thephotodiode 22 while the charge is being held at charge storage region 26before second charge transfer transistor 25 is taken high (i.e., beforereadout) may cause excess charge to be generated in the charge storageregion 26. This can cause the charge level read out from the storageregion 26 to not be an entirely accurate representation of the actualcharge generated by the photodiode 22. This may corrupt the readout fromthe pixel 30. In addition, light that is incident upon the chargestorage region 26 instead of the photodiode 22 is not converted tocharge by the photodiode 22, rendering the readout from the photodiode22 unrepresentative of the actual amount of light incident upon thepixel 30.

While backside illuminated image sensors (i.e., image sensors in whichmetal routing structures for the pixels are beneath the photodiodes suchthat light does not pass through the metal routing structures beforereaching the photodiodes) generally provide higher quantum efficiencythan frontside illuminate image sensors (i.e., image sensors in whichmetal routing structures for the pixels are between the photodiodes andthe microlenses such that light does pass through the metal routingstructures before reaching the photodiodes), a backside illuminatedimage sensor operating in a global shutter scheme is generally moresusceptible to light penetration and absorption in and around the chargestorage region 26 than is a frontside illuminate image sensors operatingin a global shutter scheme. This can lead to backside illuminated globalshutter image sensors having a lower global shutter efficiency than afrontside illuminated global shutter image sensor. Reducing the amountof light that reaches the charge storage region 26 may increase theglobal shutter efficiency of backside illuminated sensors.

A cross-sectional side view of a pixel 30 is shown in FIG. 4. Pixel 30may include a layer of silicon 46 in which photodiode 22 and chargestorage region 26 may be formed. A dielectric layer 58 (sometimesreferred to herein as an interconnect layer) beneath silicon layer 46may include a metal layer(s) 60 (sometimes referred to herein asconductive paths or metal routing structures) that form conductivestructures in pixel 30 of the type shown in FIG. 3 (e.g., pixelstructures such as transfer transistors, reset transistors, row selecttransistors, source-follower transistors, pixel power supply voltagelines, ground power supply voltage lines, pixel readout lines, columnoutput lines, etc.). A microlens 40 (sometimes referred to herein as alens) may be formed over photodiode 22 to help direct incident lightonto photodiode 22. A color filter element 62 may be formed between thelayer of silicon 46 and the microlens 40.

In order to increase the amount of incident light that reachesphotodiode 22 and minimize the amount of light that is incident uponcharge storage region 26, pixel 30 may be provided with a microlensarray 44 (sometimes referred to herein as a diffractive lens array, anin-pixel lens array, a diffractive microlens array, a group ofmicrolenses, or a set of microlenses). Microlens array 44 may includemultiple individual microlens structures 44-1, 44-2, and 44-3. As shownin FIG. 4, microlens array 44 may have a central microlens 44-1 that hasa first height (thickness) and a first diameter (width) and one or moreperipheral microlenses 44-2 and 44-3. Peripheral microlenses 44-2 and44-3 may each have second heights (thicknesses) that are less than thefirst height of central microlens 44-1 and may each have seconddiameters (widths) that are less than the first diameter of centralmicrolens 44-1. The heights and diameters of peripheral microlenses 44-2and 44-3 may be the same or may be different. A microlens array of thetype shown in FIG. 4 may help further direct light that has alreadypassed through microlens 40 away from charge storage region 26 andtowards photodiode 22. Although FIG. 4 shows only one pixel, this ismerely illustrative. If desired, multiple pixels or every pixel in array20 may be provided with a separate respective microlens 40 and aseparate respective microlens array 44. In such an arrangement, themicrolenses 40 may collectively be referred to as a microlens layer,lenses, or a lens layer. The microlens arrays 44 may collectively bereferred to as a microlens layer or a microlens array layer. Byconcentrating light more effectively towards the photodiodes 22 in therespective pixels in the array (e.g., away from storage regions 26),micro lens arrays 44 may improve the global shutter efficiency of thepixels.

If desired, a planarizing layer 42 may be provided between microlens 40and microlens array 44. Planarizing layer 42 may provide a planarsurface on which microlens 40 can be formed. Planarizing layer 42 is,however, merely optional. If desired, planarizing layer 42 may beomitted, and microlens 40 may be formed directly on the surfacemicrolens array 44 (e.g., the material for microlens 40 may fill in theuneven surfaces and gaps between the microlenses in array 44).

In the illustrative example of FIG. 4, color filter element 62 isbetween microlens 40 and microlens array 44. This, however, is merelyillustrative. If desired, color filter element 62 may be formed betweensilicon layer 46 and microlens array 44.

Materials suitable for microlens 40 and microlens array 44 includesilicon oxynitride, silicon nitride, tantalum oxides such as tantalumpentoxide, and other dielectric materials. In general, the material usedfor microlens 40 and microlens array 44 should have a sufficiently highrefractive index (e.g., a refractive index in the range of about 1.4 to4.0) to redirect light away from charge storage region 26 and towardsphotodiode 22. In one illustrative example, microlens 40 may have arefractive index of approximately 1.4. Microlens array 44 may have arefractive index of approximately 1.8. If present, planarizing layer 42may be formed of a dielectric material similar to those mentioned abovein connection with microlens 40 and microlens array 44, and may have arefractive index between that of microlens 40 and microlens array 44. Inanother suitable arrangement, microlenses in array 44 may have an indexof refraction of approximately 2, while the layers of materialsurrounding array 44 (e.g., microlens 40 and planarizing layer 42, ifpresent) may have indices of refraction of approximately 1.46-1.6. Ingeneral, the refractive indices of the materials used for microlens 40,planarizing layer 42, and microlens array 44 should increase the closerthe layer is to silicon layer 46 to ensure that the lens structures canredirect light away from charge storage region 26. If desired, eachmicrolens structure 44-1, 44-2, and 44-3 in array 44 may be made of thesame material and have the same index of refraction, or one or more ofthe microlens structures in array 44 may be formed of a differentmaterial and have a different index of refraction than the others.

In the illustrative example of FIG. 4, microlens 40 and microlens array44 are both centered over photodiode 22. While this arrangement may beused (e.g., for a pixel located at the center of array 20), it is merelyillustrative. In order to account for differences in the chief ray angleof incoming light at different locations in array 20, microlens 40and/or microlens array 44 may be shifted (sometimes referred to hereinas optical element shifting) such that their centers are not centeredover photodiode 22. For example, the centers of microlens 40 and/ormicrolens array 44 in a pixel near the edge of array 20 may be shiftedto the left or right over the center of photodiode 22. This may helpdirect light onto photodiode 22 despite a high chief ray angle. Ifdesired, microlens 40 and microlens array 44 may be shifted overphotodiode 22 by different distances (e.g., the shift of microlens 40may be greater than or less than the shift of microlens array 44).Microlens 40 and/or microlens array 44 may be shifted in any directionover photodiode 22, and may be shifted in different directions within asingle pixel 30, if desired. Color filter elements 62 may be shifted inthe same manner as microlens 40 and/or microlens array 44.

FIG. 5 is a top-down view of lens structures of the type show in FIG. 4.Although not visible in the cross-sectional side view of FIG. 4, twoother microlens structures 44-4 and 44-5 in array 44 are shown in FIG.5. These additional microlens structures 44-4 and 44-5 may be identicalto microlenses 44-2 and 44-3 as described above in terms of shape,dimensions, material, and/or index of refraction, or may be different.The microlenses in array 44 may be formed of a single piece of material,or may be formed separately. Together, the microlenses in microlensarray 44 may further redirect light from microlens 40 away from chargestorage region 26 and towards photodiode 22.

In the illustrative example of FIG. 5, microlens 40 is shown as having adiameter equal to the total combined diameters of microlens structures44-1, 44-2, and 44-3. This, however, is merely illustrative. If desired,microlens 40 may have a diameter that is greater than the combineddiameters of the microlenses in array 44, or may have a smaller diameterthan these combined diameters.

The number of microlenses in array 44 is merely illustrative. Ingeneral, an array of the type shown in FIG. 5 may include anywhere fromtwo to one hundred, one thousand, ten thousand, or more microlenses. Insome illustrative examples, an array of microlenses 44 may include five,seven, nine, eleven, thirteen, fifteen, or more microlenses.

The size of the microlenses in array 44 is merely illustrative.Microlenses in array 44 may have diameters (widths) ranging from 2microns down to sub-wavelength dimensions such approximately 500nanometers. In one illustrative arrangement, microlens 40 may have adiameter of approximately 5 microns, central microlens 44-1 may adiameter of approximately 2 microns, and peripheral microlenses 44-2,44-3, 44-4, and 44-5 may have diameters of approximately 1.4 microns. Ifdesired, however, all of the microlenses in array 44 may have the samediameter, or peripheral microlenses 44-2, 44-3, 44-4, and 44-5 may havelarger diameters than central microlens 44-1. The heights (thicknesses)of microlenses in array 44 may also vary. In one example, peripheralmicrolenses 44-2, 44-3, 44-4, and 44-5 may be thinner than centralmicrolens 44-1. If desired, however, all of the microlenses in array 44may have the same thickness, or peripheral microlenses 44-2, 44-3, 44-4,and 44-5 may have be thicker than central microlens 44-1. The radius ofcurvature of microlenses in array 44 may also vary. In one example,central microlens 44-1 may have a larger radius of curvature than theradius of curvature of each of the peripheral microlenses 44-2, 44-3,44-4, and 44-5. If desired, however, all of the microlenses in array 44may have the same radius of curvature, or peripheral microlenses 44-2,44-3, 44-4, and 44-5 may each have a radius of curvature that is largerthan the radius of curvature of central microlens 44-1.

The spatial arrangement of microlenses in array 44 is merelyillustrative. If desired, peripheral microlenses may completely surroundcentral microlens 44-1. In another example, one or all of the peripheralmicrolenses may be rotated about the center of microlens 44-1 byanywhere from one degree up to three hundred and sixty degrees. Ifdesired, there may be no central microlens in array 44. Microlenses inarray 44 may be arranged in rows and columns (e.g., a two-by-two array,a three-by-three array, etc.), arranged in a triangular pattern,arranged in an elliptical pattern, or arranged in other suitableconfigurations. The symmetry and centering of micro lens pattern 44within the pixel may be chosen to correspond with the symmetry of thearrangement of photo diode 22 and storage area 26 in a pixel for optimumlight concentration on photo diode 22 and away from storage area 26.Micro lenses 44 may be formed to overlap each other in part when formedin multiple subsequent reflow process steps.

The circular (semi-spherical) shape of microlenses in array 44 is merelyillustrative. In general, microlenses 44 may have any suitable shape.From a top-down view of the type shown in FIG. 5, microlenses in array44 may have a square shape, a rectangular shape, a square shape withrounded edges, a triangular shape, an elliptical shape, an asymmetricshape, or other suitable shapes. When viewed from a cross-sectional sideview of the type shown in FIG. 4, microlenses 44 may have a triangularcross-sectional profile, a square cross-sectional profile, a rectangularcross-sectional profile, may have a rounded cross-sectional profile(e.g., of the type shown in FIG. 4), or may have other suitablecross-sectional profiles. Microlenses 44 may include concave lenses,convex lenses, or have optical surfaces with other curvatures.

Due to the tendency of light of different wavelengths to refract atdifferent angles, the number, size, configuration, and shape ofmicrolenses in array 44 may be different for each color of pixel 30 inpixel array 20. For example, red pixels in array 20 may incorporate afirst type of microlens array 44, green pixels in array 20 mayincorporate a second type of microlens array that is different than thefirst type, and blue pixels in array 20 may incorporate a third type ofmicrolens array that is different than the first and/or second types.

If desired, multiple layers of microlens arrays may be incorporated intoa single pixel 30. A cross-sectional side view of a pixel 30 having suchan arrangement is shown in FIG. 6. Each of the microlenses in primarymicrolens array 44 (i.e., 44-1, 44-2, and 44-3, as well as 44-4 and44-5, although not shown in FIG. 6) has an additional secondarymicrolens array (sometimes referred to herein as a secondary microlenslayer or a second microlens layer) formed beneath it. In the example ofFIG. 6, microlens 44-1 overlaps and directs light onto microlens array54, microlens 44-2 overlaps and directs light onto microlens array 52,and microlens 44-3 overlaps and directs light onto microlens array 56.In general, secondary microlens arrays may be formed of the samematerials as those described above in connection with primary microlensarray 44 (sometimes referred to herein as a primary microlens layer or afirst microlens layer), and should have a higher index of refractionthan microlens array 44. Secondary microlens arrays may also vary interms of the number of microlenses they contain, the size of the lensesand the array as a whole, the spatial arrangement of the lenses in thearray, the shape of the lenses, and the shape of the array as a whole asdescribed above in connection with microlens array 44. The number ofmicrolenses in the array, the size of the lenses and the array as awhole, the spatial arrangement of the lenses in the array, the shape ofthe lenses in the array, and the shape of the array as a whole may bethe same as or different than microlens array 44. If desired, optionalplanarization layer 50 may be formed over microlens arrays 52, 54,and/or 56 in the same manner as planarization layer 42.

Although two layers of microlens arrays are shown in FIG. 6, this ismerely illustrative. If desired, a single pixel 30 may include a primarymicrolens array, a secondary microlens array, a tertiary microlensarray, and a quaternary microlens array, each having a nestedconfiguration of the type described above in connection with primarymicrolens array 44 and secondary microlens arrays 52, 54, and 56. Ifdesired, five, ten, fifteen, or more layers of in-pixel microlenses maybe incorporated into a single pixel 30.

In the illustrative example of FIG. 6, color filter element 62 is shownbetween microlens 40 and primary microlens array 44. This is merelyillustrative. If desired, color filter element 62 may be between primarymicrolens array 44 and the secondary microlens arrays 52, 54, and 56. Ingeneral color filter elements 62 may be formed between any two lens ormicrolens layers.

FIG. 7 shows in simplified form a typical image capture and processorsystem 1800, such as a digital camera, which includes an imaging device2000 (e.g., an imaging device 2000 such as image sensor 16 of FIGS. 1-6employing pixels 30 having in-pixel microlens arrays). The processorsystem 1800 is exemplary of a system having digital circuits that couldinclude imaging device 2000. Without being limiting, such a system couldinclude a computer system, still or video camera system, scanner,machine vision, vehicle navigation, video phone, surveillance system,auto focus system, star tracker system, motion detection system, imagestabilization system, and other systems employing an imaging device.

The image capture and processor system 1800 generally includes a lens1896 for focusing an image on pixel array 20 of device 2000 when ashutter release button 1897 is pressed, central processing unit (CPU)1895, such as a microprocessor which controls camera and one or moreimage flow functions, which communicates with one or more input/output(I/O) devices 1891 over a bus 1893. Imaging device 2000 alsocommunicates with the CPU 1895 over bus 1893. The system 1800 alsoincludes random access memory (RAM) 1892 and can include removablememory 1894, such as flash memory, which also communicates with CPU 1895over the bus 1893. Imaging device 2000 may be combined with the CPU,with or without memory storage on a single integrated circuit or on adifferent chip. Although bus 1893 is illustrated as a single bus, it maybe one or more busses or bridges or other communication paths used tointerconnect the system components.

In accordance with various embodiments, a pixel may include a layer ofsilicon, a photodiode formed in the layer of silicon, a charge storageregion formed in the layer of silicon, a microlens that directs lightonto the photodiode, and a microlens array interposed between themicrolens and the photodiode. The microlens array may direct light thathas passed through the microlens away from the charge storage region.The pixel may include a dielectric layer. The layer of silicon may beinterposed between the dielectric layer and the microlens array. Thepixel may include conductive paths in the dielectric layer. Electricalcharge generated by the photodiode in response to incident light may beread out on the conductive paths. The microlens array may include acentral microlens and peripheral microlenses that surround the centralmicrolens. The central microlens may have a first diameter and each ofthe peripheral microlenses may have a second diameter that is less thanthe first diameter. The microlens formed over the microlens array mayhave a third diameter that is greater than the first and seconddiameters. The microlens formed over the microlens array may have afirst index of refraction, and the central microlens and each of theperipheral microlenses in the microlens array may have a second index ofrefraction that is greater than the first index of refraction. Themicrolens array may be a primary microlens array. The pixel may furtherinclude a secondary microlens array between the primary microlens arrayand the photodiode. Charge generated by the photodiode in response toincident light may be read out in a global shutter scheme. The pixel mayinclude a planarization layer between the microlens and the microlensarray.

In accordance with various embodiments, a pixel array may include alayer of silicon. Each respective pixel in the array may include aphotodiode and a charge storage region formed in the layer of silicon.The pixel array may include a layer of lenses. Each respective pixel inthe array may include a single lens in the layer of lenses that overlapsthe photodiode in the respective pixel and directs light onto thephotodiode in the respective pixel. The pixel array may include a layerof microlenses between the layer of silicon and the layer of lenses.Each respective pixel in the array may include a group of microlenses inthe layer of microlenses that overlap the single lens in the respectivepixel and direct light away from the charge storage region in therespective pixel. The group of microlenses in each respective pixel inthe array may include a first microlens having a first radius ofcurvature and a second microlens having a second radius of curvaturethat is different than the first radius of curvature. The group ofmicrolenses in each respective pixel in the array may include a thirdmicrolens. The first microlens in each respective pixel may be a centralmicrolens at a center of the group of microlenses in each respectivepixel. The second microlens and the third microlens in each respectivepixel may be peripheral microlenses that surround the central microlens.The lens in each respective pixel in the array may have a first index ofrefraction. The first microlens and the second microlens in eachrespective pixel in the array may have a second index of refraction thatis greater than the first index of refraction. The layer of microlensesmay be a first layer of microlenses. The pixel array may include asecond layer of microlenses between the layer of silicon and the firstlayer of microlenses. Each respective pixel in the array may include agroup of microlenses in the second layer of microlenses that overlapsthe group of microlenses in the first layer of microlenses in therespective pixel. Each respective pixel in the array may include moremicrolenses from the second layer of microlenses than from the firstlayer of microlenses. The pixel array may include a layer of colorfilter elements. A first pixel in the array may include a first colorfilter element that passes light of a first color and a second pixel inthe array may include a second color filter element that passes light ofa second color that is different than the first color. A first group ofmicrolenses in the layer of microlenses in the first pixel may have afirst index of refraction. A second group of microlenses in the layer ofmicrolenses in the second pixel may have a second index of refractionthat is different than the first index of refraction. The first group ofmicrolenses in the layer of microlenses in the first pixel may have afirst diameter. The second group of microlenses in the layer ofmicrolenses in the second pixel may have a second diameter that isdifferent than the first diameter.

In accordance with various embodiments, a system may include a centralprocessing unit, memory, input-output circuitry, and an image sensorthat includes a pixel array. Each pixel in the array may include aphotodiode formed in a layer of silicon, a charge storage node formed inthe layer of silicon, a microlens that directs light onto thephotodiode, and a group of diffractive lenses between the microlens andthe layer of silicon. The group of diffractive lenses may direct lightthat has passed through the microlens to the photo diode and away fromthe charge storage node. The image sensor may be a backside illuminatedimage sensor. The image sensor may operate in a global shutter mode.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention. Theforegoing embodiments may be implemented individually or in anycombination.

1. A pixel, comprising: a substrate; a photodiode formed in thesubstrate; a charge storage region formed in the substrate; a microlensthat directs light onto the photodiode; and a microlens array interposedbetween the microlens and the photodiode, wherein the microlens arraycomprises: a central microlens having a first diameter; and a peripheralmicrolens having a second diameter that is less than the first diameter.2. The pixel defined in claim 1, further comprising: a dielectric layer,wherein the substrate is interposed between the dielectric layer and themicrolens array; and conductive paths in the dielectric layer on whichelectrical charge generated by the photodiode in response to incidentlight is read out.
 3. The pixel defined in claim 1, wherein themicrolens array comprises: a plurality of additional peripheralmicrolenses, wherein the plurality of additional peripheral microlensesand the peripheral microlens surround the central microlens.
 4. Thepixel defined in claim 3, wherein each of the plurality of additionalperipheral microlenses has a diameter that is less than the firstdiameter.
 5. The pixel defined in claim 1, wherein the microlens has athird diameter that is greater than the first and second diameters. 6.The pixel defined in claim 1, wherein the microlens has a first index ofrefraction and wherein the central microlens and the peripheralmicrolens each has a index of refraction that is greater than the firstindex of refraction.
 7. The pixel defined in claim 1, wherein themicrolens array is a primary microlens array, the pixel furthercomprising: a secondary microlens array interposed between the primarymicrolens array and the photodiode.
 8. The pixel defined in claim 1,wherein charge generated by the photodiode in response to incident lightis read out in a global shutter scheme.
 9. The pixel defined in claim 1,further comprising: a planarization layer interposed between themicrolens and the microlens array.
 10. A pixel array comprising: a layerof silicon, wherein each respective pixel in the array includes aphotodiode and a charge storage region formed in the layer of silicon; alayer of lenses, wherein each respective pixel in the array includes asingle lens in the layer of lenses that overlaps the photodiode in therespective pixel and directs light onto the photodiode in the respectivepixel; and a layer of microlenses interposed between the layer ofsilicon and the layer of lenses, wherein each respective pixel in thearray includes a plurality of microlenses in the layer of microlensesthat overlap the single lens in the respective pixel and direct lightaway from the charge storage region in the respective pixel and whereinthe plurality of microlenses in each respective pixel in the arraycomprises: a first microlens having a first radius of curvature; and asecond microlens having a second radius of curvature that is differentthan the first radius of curvature.
 11. (canceled)
 12. The pixel arraydefined in claim 10, wherein the plurality of microlenses in eachrespective pixel in the array comprises: a third microlens, wherein thefirst microlens in each respective pixel is a central microlens at acenter of the plurality of microlenses in each respective pixel, andwherein the second microlens and the third microlens in each respectivepixel are peripheral microlenses that surround the central microlens.13. The pixel array defined in claim 10, wherein the lens in eachrespective pixel in the array has a first index of refraction, andwherein the first microlens and the second microlens in each respectivepixel in the array has a second index of refraction that is greater thanthe first index of refraction.
 14. The pixel array defined in claim 10,wherein the layer of microlenses is a first layer of microlenses, thepixel array further comprising: a second layer of microlenses interposedbetween the layer of silicon and the first layer of microlenses, whereineach respective pixel in the array comprises: a plurality of microlensesin the second layer of microlenses that overlap the plurality ofmicrolenses in the first layer of microlenses in the respective pixel.15. The pixel array defined in claim 14, wherein each respective pixelin the array comprises more microlenses that are in the second layer ofmicrolenses than are in the first layer of microlenses.
 16. The pixelarray defined in claim 10, further comprising: a layer of color filterelements, wherein a first pixel in the array includes a first colorfilter element that passes light of a first color, wherein a secondpixel in the array includes a second color filter element that passeslight of a second color that is different than the first color, whereina first plurality of microlenses in the layer of microlenses in thefirst pixel have a first index of refraction, and wherein a secondplurality of microlenses in the layer of microlenses in the second pixelhave a second index of refraction that is different than the first indexof refraction.
 17. The pixel array defined in claim 10, furthercomprising: a layer of color filter elements, wherein a first pixel inthe array includes a first color filter element that passes light of afirst color, wherein a second pixel in the array includes a second colorfilter element that passes light of a second color that is differentthan the first color, wherein a first plurality of microlenses in thelayer of microlenses in the first pixel have a first diameter, andwherein a second plurality of microlenses in the layer of microlenses inthe second pixel have a second diameter that is different than the firstdiameter.
 18. A system, comprising: a central processing unit; memory;input-output circuitry; and an image sensor comprising: a pixel array, apixel in the pixel array comprising: a photosensitive element formed ina substrate layer; a charge storage node formed in the substrate layer;a microlens that directs light onto the photosensitive element; and aplurality of diffractive lenses interposed between the microlens and thesubstrate layer, wherein the plurality of diffractive lenses comprises aplurality of lenses that overlap the pixel and that each has an index ofrefraction that is greater than an index of refraction of the microlens.19. The system defined in claim 18, wherein the image sensor is abackside illuminated image sensor.
 20. (canceled)
 21. The system definedin claim 18, wherein the plurality of lenses comprises a central lensand a plurality of peripheral lenses that surround the central lens. 21.The system defined in claim 21, wherein each peripheral lens in theplurality of peripheral lenses has a size that is different from a sizeof the central lens.